Translucent polycarbonates can be manufactured for internally illuminated signs, protective light diffusers for fluorescent, incandescent or gaseous discharge lamps, translucent window lights, and the like.
It is known in the art that polycarbonate resins can be rendered translucent by the use of one or more inorganic additives such as titanium dioxide, zinc oxide, lead carbonate, lithopone, talc, etc., either alone or in combination. However, all of these inorganic salts produce translucent formulations which are not completely satisfactory as they are deficient in one or more properties. For example, at the high temperatures normally encountered during manufacturing, many of these inorganic light diffusers are chemically reactive and degrade the desirable physical and optical properties of the polycarbonate composition.
In making a translucent formulation, many variables are to be considered; e.g., the color of the light reflected from the surface of a molded part, the color and intensity of the light transmitted through the molded part, and particularly, the radial energy distribution of light transmitted through a molded part from a parallel beam of light. An ideally diffuse part, when illuminated by a narrow, parallel beam of light, will exhibit a spherical energy distribution on the side opposite the impinging beam, a property especially important when it is desired to obscure the light source and present a uniformly illuminated surface. The more this distribution departs from the spherical (that is, the greater the proportion of light propagated specularly through the part), the less ideal is the part as a diffuser.
This property of a light diffuser can be measured conveniently on a G.E. Recording Spectrophotometer by measuring the diffuse light (T.sub.d) transmitted through a sample, the light transmitted specularly being absorbed by a black velvet light trap placed in the external sample port, and comparing it with the total light (T.sub.t) transmitted through the sample. The ratio, T.sub.d /T.sub.t, is called the diffusivity, D, of the sample. The closer this ratio approaches 1.0, the most closely the sample approaches an ideal diffuser.
In addition, a translucent composition must also be thermally stable, retain its desirable physical toughness and impact strength and be easily processable.